#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
	int n;
	cin >> n;
	double a, b, fn;
	a = b = fn = 0;
	a = pow((1 + sqrt(5)) / 2.0, n);
	b = pow((1 - sqrt(5)) / 2.0, n);
	fn = (a - b) / sqrt(5);
	cout << fixed << setprecision(2) << fn ;
	return 0;
}
